It used to be that there were only two forum technologies: (1) Flat -- Quora (2) Threaded -- Reddit.

Flat designs suffer from "the scattered content problem." Specifically, the content in flat designs is sorted by date, which scatters similar topics across the conversation, making it hard to find all the contributions on topics of interest.

Threaded designs suffer from overly complex layouts that are difficult to navigate. Typical threaded designs use indents to denote relationships. This becomes problematic when screen real estate is limited, additionally so when you must scroll horizontally.

Because of the way the content is organized in either designs it can be difficult for users to digest the content and contribute effectively, especially as the discussion grows.

http://www.drewgl.com... has a new forum technology that seems to avoid both these issues. It uses flat, non time-centric discussion board that is navigated using an interactive visual feedback display. As far as I know, no other forum design has anything like it.